IP uses Packets called IP packets to carry information. Every IP packet is a single unit of information and besides data it carries information to determine where to send the packet. IP determines where to send packets to by looking at the destination IP address.
The IPv4 packet header has quite some fields. In this bog we’ll take a look at them and I’ll explain what everything is used for. Take a look at this picture:
If you learned about the OSI Model and encapsulation / decapsulation you know that when two computers on the LAN want to communicate with each other the following will happen:
Cisco IOS routers and layer 3 switches can be configured as DHCP server. It’s quite easy to do this and in this blog we’ll discuss how to do this and how to verify our configuration.
DHCP is often used for hosts to automatically assign IP addresses and uses 4 different packets to do so. Since a host doesn’t have an IP address to start with, we use broadcast messages on the network that hopefully end up at a DHCP server.
In this blog, we are going to take a look at our transport protocols TCP and UDP. If you know about IP and IP packets you know that we require a transport protocol to send our IP packets.
TCP (Transmission Control Protocol) is a reliable transport protocol as it establishes a connection before sending any data and everything that it sends is acknowledged by the receiver. In this blog we will take a closer look at the TCP header and its different fields. Here’s what it looks like:
TCP (Transmission Control Protocol) is a connection oriented protocol which means that we keep track of how much data has been transmitted. The sender will transmit some data and the receiver has to acknowledge it. When we don’t receive the acknowledgment in time then the sender will re-transmit the data.
TCP is one of those protocols that we usually don’t think about too much. As network engineers we are busy working with network devices like routers or switches. TCP is one of those protocols that is used most between hosts or servers and it works without giving it much thought. It establishes connections, transmits data, sends acknowledgments and when something goes wrong…it retransmits it.
ICMP (Internet Control Message Protocol) is a network protocol used for diagnostics and network management. A good example is the “ping” utility which uses an ICMP request and ICMP reply message. When a certain host of port is unreachable, ICMP might send an error message to the source. Another example of an application that uses ICMP is traceroute.
Let’s look at the following topology:
Asymmetric routing is not uncommon and it doesn’t always cause issues. There are however a number of scenarios where it could cause problems. For example:
Routers are often used on the edge of our network where we use them for Internet access or connectivity to other sites. The Internet is a bad place so routers are vulnerable to a number of attacks.
To mitigate these attacks you should have a (formal) document that describes how you are going to deal with these attacks and how you will protect your routers. This document is called a router security policy. The advantage of using a security policy is that all your routers will have the same consistent configuration.